I. Field of the Invention
The present invention relates to low-Earth orbit (LEO) satellite communication systems. More specifically, the present invention relates to using spatial multiplexing to increase the bandwidth of the communication links between the satellites and the Earth stations in such a system.
II. Related Art
A typical satellite-based communication system comprises at least one terrestrial base station (referred to as a "gateway"), at least one user terminal (for example, a mobile or hand-held portable telephone), and at least one satellite for relaying communication signals between the gateway and the user terminal. The gateway provides links from a user terminal to other user terminals or communication systems, such as a terrestrial telephone system.
One of the limitations imposed on satellite communications systems is the allocated frequency bandwidth. Various governmental agencies and standards associations allocate bandwidth in the total signal spectrum taking into consideration various political and technical constraints. One type of allocation granted for use in Low Earth Orbit satellite based communications systems is granted by the ITU (International Telecommunications Union) and uses a 200 MHz bandwidth. Given the fixed allocation constraints and an extreme unlikelihood of obtaining additional bandwidth, it is desirable to maximize the capacity of this bandwidth allocation, to thereby maximize the number of users who can utilize a given system at any time.
Conventional satellites use one large beam or beamforming antennas on the communication links between the gateway and the satellite (referred to as a "feeder link") and between the satellite and the user terminal (referred to as the "user link"). Typically, a single feeder link is used so that a single path couples each satellite with all gateways within view of the satellite. On the user link side, multiple links divide the coverage area of the satellite (referred to as a "footprint") into a plurality of geographic regions (referred to as "cells"). Each cell is illuminated by one of the user link beams. Each beam uses the full bandwidth allocated to the user link. The resulting bandwidth available for user terminal communication is given by the product of the user link bandwidth and the number of user link beams.
As the demand for such satellite-based communication services increases, so must the capacity of such systems. Given that user bandwidth allocations are at present inflexible, one way to increase the capacity of the user link is simply to increase the number of user link beams. However, any increase in user link capacity must be accompanied by an increase in the capacity of the communication link between the satellite and the gateways.
It is, therefore, desirable to provide a system and method for increasing the capacity of a satellite communications system in which the bandwidth cannot be increased.